I. Field of the Invention
This invention relates to the refining of lithium-containing aluminum scrap.
II. Description of the Prior Art
Aluminum-lithium alloys are used in the aircraft industry and for other specialized markets and large amounts of scrap are produced during the manufacture of specialized parts from the alloys. Recycling of the scrap is economically desirable but these particular alloys present difficult problems when they enter the scrap market. The alloys cannot merely be re-melted and used again for the same purposes because they have picked up iron and other impurities which adversely affect the metallurgical properties of the alloys. However, the alloys cannot be used with other aluminum scrap because the lithium is harmful to more conventional aluminum alloys, for example the casting alloys which are the normal end-product of aluminum scrap. Moreover, lithium is expensive and should be recovered, if possible.
Lithium may be removed from Al-Li alloys by chlorination to convert all of the lithium to LiCl, but this procedure is wasteful of energy and it involves the use of chlorine on a massive scale, which is environmentally hazardous.
Another possible way of removing lithium from the scrap is by electrolysis using molten scrap as an anode and a lithium chloride based electrolyte. However, it is known that lithium is quite soluble in lithium chloride at the normal cell operating temperatures of about 700.degree. C. Nakajima et al in "Miscibility of Lithium with Lithium Chloride and Lithium Chloride-Potassium Chloride Eutectic Mixture", Bulletin of the Chemical Society of Japan, Vol. 47(8), 2071-2072 [1974], show that the solubility of lithium is about 0.8 mole % Li at 700.degree. C. (0.27 equivalents Li/litre). Such solubility would be expected to dramatically reduce the cell current efficiency. For comparison, the solubility of aluminum in the electrolyte of an aluminum reduction cell is about 0.07 equivalents/litre and this gives rise to a 10% reduction in current efficiency. Lithium, being four times as soluble, could be expected to give a 40% reduction, which would be economically unattractive.
This potential problem would be expected to be particularly pronounced when pure lithium is collected at the cathode. The problem could perhaps be alleviated by forming an Li-Al alloy at the cathode, which would be expected to reduce the activity of lithium to the order of 0.03, and consequently would be expected to reduce the lithium solubility proportionally. However, even in this case, there would be a problem in determining when the optimum removal of lithium from the anode had taken place. This is important because lithium scrap is by its nature of inconsistent composition, so the amount of Li is not known in advance. If electrolysis of aluminum from the anode takes place, aluminum chloride is produced in the electrolyte and this is undesirable because aluminum chloride is volatile. Moreover, there is no economic advantage in transferring aluminum from the anode to the cathode.
For these reasons it has not been apparent that refining to pure lithium is practical at all nor that refining to Al-Li alloy is effectively controllable.